It’s my personal theory that the “Fourth War” includes the Garrosh one. Basically everything from Varian declaring war in the Undercity in Wrath up until now I think could be considered one conflict.
Although, the Hellscream war wasn’t even the 4th conflict between the Alliance and Horde anyway. The Invasion of Dreanor is treated as its own war, but not named the “Third War”, nor was the war on the interment camps Orgrim and Thrall led inbetween the Second and Third Wars. Then there was the Invasion of Durotar, another Alliance versus Horde conflict not counted.
I don’t know what identifies this one as the Fourth War though. Sargeras’ Sword? The rest of the wars were driven by demon influence before.